1. Field of the Invention
The present invention relates to a process for preparing fluorocarboxylic acids. More particularly, it relates to a process for preparing fluorocarboxylic acids in high purity and in high yield from fluorocarboxylates. The invention relates specifically to a process for preparing C4-14, and especially C7-9, fluorocarboxylic acids, including fluoroalkylcarboxylic acids, and particularly perfluoroalkylcarboxylic acids.
2. Description of the Related Art
Fluorocarboxylic acids, such as C8 fluorocarboxylic acids and their salts, are known to have good surface activity. These fluorocarboxylic acids, as well as ammonium salts and alkali metal salts thereof, are widely used as polymerization emulsifying agents in the polymerization of fluoroolefins such as tetrafluoroethylene. Moreover, given the generally high cost of such fluorocompounds and their impact on the environment, it is desirable that they be recovered, purified and reused to the greatest possible extent.
One method that has been proposed for recovering and purifying such fluorocompounds involves preparing (or regenerating) fluorocarboxylic acid from a fluorocarboxylate-containing aqueous solution. In this method, in a tank-type reactor, sulfuric acid is used to carry out an acidification reaction on a fluorocarboxylate-containing aqueous solution, giving an acidification solution. Next, the acidification solution is heated and then liquid layers are separated to obtain a fluorocarboxylic acid-containing organic phase and an aqueous phase. The organic phase is recovered, then washed with an aqueous sulfuric acid solution to recover the fluorocarboxylic acid. If necessary, the recovered fluorocarboxylic acid is subsequently purified by distillation (see U.S. Pat. No. 6,281,374).
One problem with this related art method is the loss of fluorocarboxylic acid with liquid-liquid separation following the acidification reaction. Moreover, this related art process uses as the starting material an aqueous solution having a fluorocarboxylate concentration of about 5 to 40 wt %, preferably 5 to 30 wt %, more preferably 10 to 20 wt %. If the fluorocarboxylate concentration is below the desired range within the limits disclosed above, the solution can be concentrated by evaporation or other means.
As can be seen from the above, related art processes give rise to a loss of fluorocarboxylic acid. Moreover, in cases where the fluorocarboxylic acid preparation process includes enrichment, acidification, liquid-liquid separation and subsequent recovery, it is not easy to make the production process continuous.